1. Field of the Invention
This invention relates to an image forming apparatus such as a copier or a printer using the electrophotographic process or the electrostatic recording process.
2. Related Background Art
FIG. 9 of the accompanying drawings schematically shows the construction of a color image forming apparatus. This color image forming apparatus has a digital color image reader portion in the upper portion thereof, and a digital color image printer portion in the lower portion thereof.
In the digital color image reader portion, and original 30 is placed on an original supporting glass table 31, and a reflected optical image from the original 30 exposed and scanned by an exposure lamp 32 is condensed on a full color sensor 34 by a lens 33 to thereby obtain a color resolved image signal.
The color resolved image signal is processed by a video processing unit, not shown, via an amplifying circuit, not shown, and is delivered to the digital color image printer portion.
In the digital color image printer portion, a photosensitive drum 1 which is an image hearing member is supported for rotation in the direction of arrow R1. Around this photosensitive drum 1, there are disposed a pre-exposure lamp 11, a corona charger 2, a laser beam exposing optical system 3, a potential sensor 12, four developing devices 4y, 4c, 4m, 4bk, a density detecting sensor 13 as density detecting means, a transferring apparatus 5 and a cleaning device 6.
The color resolved image signal from the digital color image reader portion is inputted to the laser beam exposing optical system 3, and the color resolved image signal is converted into an optical signal by a laser output portion, not shown, whereafter a laser beam is reflected by a polygon mirror 3a, and is converted into an optical image E passing through a lens 3b and a mirror 3c and linearly scanning (raster scanning) the surface of the photosensitive drum 1.
During the image formation in the digital color image printer portion, the photosensitive drum 1 is first rotated in the direction of arrow R1, and the surface thereof is deelectrified by the pre-exposure lamp 11, where after it is uniformly charged by the corona charger 2, and the optical image E is applied to this charged surface for each resolved color to thereby form an electrostatic latent image.
Next, a predetermined developing device is operated for each resolved color to thereby develop and visualize the electrostatic latent image on the photosensitive drum 1, and an image by a toner as a developer having resin as a base substance is formed on the photosensitive drum 1.
Here, the developing devices 4y, 4c, 4m and 4bk are adapted to be selectively moved toward the photosensitive drum 1 in conformity with respective resolved colors by the operation of eccentric cams 24y, 24c, 24m and 24bk. 
Further, the toner image as developer image on the photosensitive drum 1 are transferred to a recording material as a sheet supplied from a recording material cassette 7 to a position opposed to the photosensitive drum 1 through a conveying system and the transferring apparatus.
The transferring apparatus 5 has, in the present example, a transferring drum 5a, a transferring charger 5b, an adsorbing charger 5c for electrostatically adsorbing the recording material, an adsorbing roller 5g opposed to the adsorbing charger 5c, an inner side charger 5d and an outer side charger 5e, and a recording material carrying sheet 5f formed of a dielectric material is cylindrically integrally extended in the opening area of the peripheral surface of the transferring drum 5a journalled so as to be rotatively driven.
As the recording material carrying sheet 5f, use is made of a dielectric material sheet such as polycarbonate film.
As the transferring drum 5a is rotated, the toner image on the photosensitive drum 1 are transferred onto the recording material carried on the recording material carrying sheet 5f, by the transferring charger 5b. 
As described above, a desired number of color images are transferred to the recording material adsorbed and conveyed by the recording material carrying sheet 5f, and finally a full color image is formed on the recording material.
In the case of a four-color mode, when the transfer of the toner images of four colors to the recording material is completed in this manner, the recording material is separated from the recording material carrying sheet 5f by the action of a separating pawl 8a, a separating push-up roller 8b and a separating charger 5h, and the toner images and fixed on the recording material through a heat roller fixing device 9 and the recording material is discharged onto a tray 10.
Also, after the transfer, the photo-sensitive drum 1 has the residual toners on its surface removed by the cleaning device 6, and thereafter is again used for the image forming process.
When images are to be formed on both surfaces of the recording material, immediately after the recording material has been discharged from the fixing device 9, a conveying path changeover guide 19 is driven, and the recording material is guided to a reversal path 21a via a sheet discharge vertical path 20, whereafter the recording material is once stopped, and is caused to leave in a direction opposite to the direction in which it has been fed in with the trailing end thereof when fed in as the leading end by the reverse rotation of reversing rollers 21b, and the recording material is reversed and stocked in an intermediate tray 22. Thereafter an image is again formed on the other surface of the recording material by the image forming process.
Also, the recording material carrying sheet 5f on the transferring drum 5a is contaminated by the scattering and adherence of powder from the photosensitive drum 1, the developing devices 4, the cleaning device 6, etc., the adherence of the toners during the jam of the recording material, and sometimes the adherence of oil on the recording material during the image formation on the both surfaces of the recording material, but is cleaned by the action of a fur brush 14 and a backup brush 15 opposed to the fur brush 14 with the recording material carrying sheet 5f interposed therebetween, and an oil removing roller 16 and a backup brush 17 opposed to the oil removing roller 16 with the recording material carrying sheet 5f interposed therebetween, and thereafter is used again for the image forming process. Such cleaning is effected during pre-rotation and during post-rotation, and is effected whenever jam occurs.
In the present example, design is made such that the eccentric cam 25 of the transferring drum is operated and a cam follower 5i made integral with the transferring drum 5a is operated, whereby the gap between the recording material carrying sheet 5f and the photosensitive drum 1 can be set to a predetermined spacing at predetermined timing. For example, it is a construction in which during standby or during the OFF of power source, the spacing between the transferring drum 5a and the photosensitive drum 1 is made great and the rotation of the transferring drum 5a can be made independent of the rotative driving of the photosensitive drum 1.
Also, each developing device is provided with first and second agitating and carrying means designed to carry the toner in opposite directions. Also, a developing sleeve is disposed above the first agitating and carrying means.
In the above-described series of image forming operations, the developing device operates as follows. When the photosensitive drum 1 is rotated and the electrostatic latent image thereon arrives at a developing position, a developing bias comprising AC and DC superimposed one upon the other is applied from a developing bias voltage source to the developing sleeve, and the developing sleeve is rotated by a developing sleeve driving device, not shown, and the developing device is pressed toward the photosensitive drum 1 by a developing pressing cam to thereby visualize the electrostatic latent image by the toner.
Also, the density detecting sensor 13, as shown in FIG. 2A of the accompanying drawings, is a regularly reflected light sensor 13a as regularly reflected light detecting means using a near infrared light LED 13c as a light emitting element and a photodiode 13d as a light receiving element to detect the quantity of regularly reflected light to the light applied to the visualized toner image on the photosensitive drum 1. It controls the toner/carrier density when a two-component developer is used as the developer, from the toner density of the toner image converted from the detected quantity of regularly reflected light, or controls the charging bias by the corona charger 2.
However, in the density detecting sensor 13 which is the regularly reflected light sensor 13a according to the prior art for detecting the quantity of regularly reflected light as described above, the detection output is saturated during a density change in the high density area of the toner density, as shown in FIG. 2B of the accompanying drawings, and it has been difficult to detect the density. Therefore, the density detection control of a low density area to a medium density area has been possible, but the density detection control in a high density area near the maximum density could not be effected.
It is an object of the present invention to provide an image forming apparatus in which the detection of density from low density to high density can be effected.
It is another object of the present invention to provide an image forming apparatus comprising:
image forming means for forming a toner image;
first detecting means for detecting a quantity of regularly reflected light from the toner image; and
second detecting means for detecting a quantity of irregularly reflected light from the toner image.
Further objects of the present invention will become apparent from the following description.